Tomorrow's pre-Budget report will be an awesome test of the cleverness of Labour and Conservatives alike. As Alistair Darling empties his early Christmas sackful of fiscal goodies, and George Osborne responds with dire warnings of the price we will all pay for the Government's reckless borrowing, the two politicians will commit their respective parties starkly and irreversibly to the competing narratives that will shape the next general election. Posing as statisticians and men of finance, Darling and Osborne will, in fact, be launching two dramatically different campaigns.

The Conservative dilemma is acute. As David Cameron's immediate predecessors discovered, it is always hard for an Opposition to compete with a government bonanza, however artificial. The national minimum wage introduced by Tony Blair in 1999 was a regulation not a gift: it was business that had to foot the bill. Yet the measure had the aura of political munificence, as if New Labour were giving the nation a huge present.

One senior Cameroon describes tomorrow's PBR as the "moment of maximum difficulty", and rightly so. Mr Darling will promise spending hikes and tax cuts, while Mr Osborne will refuse to match any measures that would jeopardise his iron commitment to fiscal conservatism, balanced books and "stability". He and David Cameron know perfectly well how awkward their position will be as a Labour Chancellor looks at them across the Despatch Box and pledges both to increase spending and to let the voters keep more of their earnings.

Labour will posture as the party of Yuletide bounty, putting cash in the punters' pockets on the very eve of the festive season. The Tories will sit, grim-faced, aware that they have just become the Official Scrooge Party: "Keynesian giveways? Bah! Humbug!"

Needless to say, there are many wobbles on the Conservative side and some anger, too. Mr Osborne's internal party critics declare that he has failed conspicuously to step up to the plate: that the Tories should have done more to respond to this global crisis than drop an already frayed commitment to stick to Labour's spending plans for 2010-11. There is genuine fear, even among the Shadow Chancellor's admirers, that the Conservatives no longer look like a government-in-waiting and are fighting the last electoral war as the Prime Minister trots the globe reinventing himself as an economic messiah.

The Tory task, to borrow from Life of Brian, is to prove that Gordon is not the messiah: he's a very naughty boy. The poster unveiled by the party this weekend, depicting a Christmas present as a "tax bombshell", imports the slogan that helped win the 1992 election to a very different context, 16 years on. The message is that the beautifully wrapped Nintendo Wii and Blu-ray Player under the tree, labelled "Happy Christmas from Gordon and Alistair", have been paid for using our own credit card details and that the bill will soon be arriving in the form of unmanageable public debt and crippling tax rises.

In this respect, the true model for the Conservative position is not John Major's campaign against Neil Kinnock in 1992 but Bill Clinton's against George Bush Sr in the same year. The foundation of Clinton's strategy - other than the numinous attraction of "change" - was the promise to curb the federal deficit, which he presented persuasively as a mortal threat to America's economic prospects and the financial wellbeing of its families. Hillary Clinton called it "The Story": how an incompetent economic elite had lied to the American people about the state of the public finances, with terrible consequences.

The Cameron-Osborne strategy now depends upon their capacity to make government recklessness and the scale of public debt no less salient an issue for British voters than it was for their US counterparts in 1992. They must hammer home the cost of that borrowing to every household, making the abstractions of meaninglessly huge statistics real and comprehensible to the average voter: debt that - according to the policy institute Reform - would wipe out more than a quarter of families' disposable income.

Intimately connected with this is the question of election timing. Some of the speculation is entirely spurious spin from the Labour side, its objective purely to unnerve the Tories. But not all of it is spurious. The morning after Labour's victory in the Glenrothes by-election one breathless Cabinet Minister told me that "the big day could come any time now" - adding that the return to the fray of Lord Mandelson had given Brown the confidence to be strategically bold.

And there is no doubt that an early election would be the smart move. While the recession is still essentially something that voters fear rather than feel, the incumbent PM - bountiful, busy, in command of the field - will be an appealing figure. But once fear is replaced by genuine pain, the man in charge will start to be the focus of resentment, and his "novice" opponent Cameron rather more enticing.

The Tories are rattled by the fresh round of election chatter, but still believe, on balance, that Brown will play it long. According to one senior Shadow Cabinet member the best Gordon could hope for by going to the country early is to lead a minority government: "And he'd look pretty foolish doing that in the middle of a recession." Maybe - who, after all, wants to go to bed with Tony Blair's parliamentary strength and wake up with Jim Callaghan's? But the more powerful factor in Brown's reckoning will be his adamantine faith in his own "long-term decisions" and his belief that, with time, ever more voters will be rescued from their false consciousness and realise that they do, in fact, love Big Brother. Why interrupt this Great National Awakening by going to the country on (say) June 4, 2009?

The Tories, meanwhile, hope that there will be public awakening of a different sort. Yes, tomorrow will be hard pounding for them, as Brown and Darling roll up their sleeves and manfully pull the levers of the Heath Robinson machine that is government. Cameron and Osborne know that their obstinacy this week will be widely interpreted as inaction. Their hope is that, as the huge cost and general impotence of the Brown-Darling fiscal strategy become manifest, what is perceived as Tory inaction will come to be seen as resolve and consistency. Wait a year, the Cameroons say: let's see how macho and impressive Gordon's fiscal stimuli look then.

Be in no doubt: Osborne is taking a huge gamble. What is not true is the accusation that he is a Bullingdon bunny frozen in the headlights of history. He has thought this through, made a decision and will live by the consequences. To return to the construction used by the character in David Hare's play, it may well be that, tomorrow, the Government appears to be doing good by being clever. But every time you read the words "giveaway", "bonanza" and "much-needed cash injection" this week, remember that this is not the final act in this particular political drama. Public confidence in Gordon could yet turn to collective doubt of the sharpest sort: Gethsemane may still lie ahead.